In recent years, plastic photochromic lenses employing organic photochromic colorants have been sold for use in eyeglasses. They darken in the bright outdoors, having the same antiglare effect as high-density color lenses, but return to high transmittance when moved indoors.
The methods of providing a coating (photochromic film) containing a photochromic dye on a lens substrate, coating a photochromic film by means of a lens substrate, positioning a photochromic film between two lens substrates, and the like are employed to impart photochromic properties to plastic lenses (for example, see Japanese Unexamined Patent Publication (KOKAI) No. 2005-305306 and the English language family member thereof, US2005/0168690A1, which are expressly incorporated herein by reference in their entirety). The photochromic film employed is required to rapidly darken at high density in response to the entry of prescribed light, and rapidly fade when placed in an environment where this light is absent.
As a method of manufacturing a photochromic lens comprising a structure in which a photochromic film is present on a lens substrate, Japanese Unexamined Patent Publication (KOKAI) Heisei No. 10-231331 and the English language family member thereof, U.S. Pat. No. 5,914,174, which are expressly incorporated herein by reference in their entirety, propose a method (cast polymerization method) by which, in the course of manufacturing a plastic lens by cast polymerization, a coating liquid containing a photochromic dye is precoated on the inner surface of a mold, and curing of the lens is conducted simultaneously with formation of the photochromic film.
The darkening density and response speed of the darkening and fading of a photochromic film are thought to depend on inherent characteristics of the photochromic dye resulting from its molecular structure. Thus, the use of photochromic dyes having specific molecular structures to improve the responsiveness (response speed and darkening density) of a photochromic film to light has been examined.
However, the responsiveness (response speed and darkening density) of a conventional photochromic film to light is not necessarily satisfactory. Further improvement is needed in light responsiveness.
In the manufacturing method described in above-cited Japanese Unexamined Patent Publication (KOKAI) Heisei No. 10-231331, to ensure adhesion between the photochromic layer and the lens substrate, prior to injecting the lens monomer, the photochromic liquid that has been coated on the inner surface of the mold is in an unpolymerized or partially cured state, and once the lens monomer has been injected, the photochromic liquid and lens substrate are sequentially cured. Since this method permits the simultaneous imparting of a photochromic film during curing of the lens substrate, it affords the advantage of excellent workability. However, since the lens substrate and photochromic layer mix together, there is a problem in the form of diminished optical characteristics (clouding, formation of striae) in the lens. Further, the light responsiveness (response speed and darkening density) of the photochromic lens that is obtained by this method is not necessarily satisfactory. Still further improvement is needed in light responsiveness.